Modern circuit protection increasingly calls for circuit breakers equipped with so-called "static" or electronic trip units in lieu of the traditional thermal-magnetic or dual-magnetic trip units operating to effect automatic opening of the breaker contacts in response to overcurrent conditions ranging from light overload to heavy short circuit. Static trip units are found to be more versatile and precise in terms of selectively establishing multiple overcurrent trip pick-up levels and trip time delays. For example, currently available static trip units have the capability of selectively establishing coordinated long-time delay, short-time delay and instantaneous current pick-up levels, as well as different tolerance bands of time delays. As a consequence, the trip settings of a static trip circuit breaker can be readily tailored to a particular load so as to provide proper protection and yet avoid vexatious nuisance tripping.
Another reason for the current popularity of static trip circuit breakers is the increasing demand for ground fault protection. Since response to a ground fault condition is best handled electronically, it becomes quite practical to integrate the ground fault trip function into an overcurrent responsive electronic trip unit in contrast providing an electronic ground fault trip unit plus a traditional electromechanical trip unit.
Once a static trip circuit breaker goes into the field, it is desirable to periodically verify its continuing capability to provide the full measure of circuit protection intended for a particular application. To this end, field test sets of two types have been made available. In one type, low current, fault simulating signals are injected into the secondary circuits of the breaker phase current sensing transformers whose secondary are connected as separate inputs to the static trip unit. If the trip unit is functioning properly, it will process these fault simulating signals pursuant to initiating a trip function as though corresponding high currents of overload proportions actually flow through the breaker poles, i.e., the primaries of the breaker current transformers. Applicant's co-pending application Ser. No. 815,628, filed July 14, 1977 discloses a static trip circuit breaker field test set of this type.
With the other type of static trip circuit breaker field test set, a high current of overcurrent proportions is passed through the breaker poles to verify that the breaker will trip with the appropriate delay. When a static trip circuit breaker is equipped with integral ground fault protection, the high current from the test set must be passed in opposite directions through two of the breaker poles in series, otherwise the ground fault sensing differential current transformer within the trip unit would be unbalanced, precipitating the initiation of a ground fault trip function. If the test current is passed through but one breaker pole, the consequent tripping of the circuit breaker cannot be said to verify the operability of the overcurrent tripping capability of the trip unit, since it is quite likely the breaker tripped in response to the differential current transformer unbalance. Thus verification of the overcurrent trip settings and trip time delays established in the trip unit cannot be reliably obtained.
As noted above, the current practice in defeating a ground fault trip function is to pass the high current generated by the test set in opposite directions through two of the breaker poles in series. Since the typical high current field test set is portable in nature and operates at a relatively low voltage, the added impedance of the second breaker pole severely limits the maximum level of test current the test set can develop. As a consequence, in many situations the test set cannot generate sufficient current levels to verify the operability of the static trip circuit breaker under simulated heavy overload and short circuit conditions.
It is accordingly an object of the present invention to provide a high current static trip circuit breaker field test set which is equipped with means for defeating a ground fault trip function without having to pass the test current through two breaker poles in series.
An additional object of the present invention is to provide a ground fault defeat cable for connecting the breaker current transformers to the static trip unit pursuant to inhibiting the initiation of a ground fault trip function despite the fact that high levels of test current is passed through but a single breaker pole.
Yet another object of the present invention is to provide a ground fault defeat cable of the above character which is inexpensive to manufacture and convenient to use in the field.
Other objects of the invention will in part be obvious and in part appear hereinafter.